Chess Opening Theory/1. e4/1...e5/2. d4/2...d6

2...d6
This is an inferior way to enter a Philidor. While white's best move is still Nf3 transposing into the Philidor, it gives them the additional option of immediately entering an endgame with dxe5 dxe5 Qxd8+ Kxd8 Bc4, though practically black hasn't had trouble defending it. Black would lose the right to castle, and White would have a lead in development. According to chess365, after 5.Bc4, white wins 42.1%, 32.5% end in a draw, and 25.1% end in a black victory. White can also enter a Chekhover-like position via Nc3 exd4 Qxd4 with the intention to respond to Nc6 with the Bb5 pin or drawing the queen back to d3 or e3 and simply enjoying their space advantage.

As such, it is considered strictly inferior to the Philidor move order, because white's sideline alternatives after e4 e5 Nf3 d6 are much less aggressive.